In recent years, particularly in magnetic recording medium for video, the thickness of the medium is decreased in order to increase recording and reproducing time. However, the thickness reduction weakens the mechanical strength of the magnetic recording medium, and when it is loaded in a cassette or cartridge, an edge portion contacts a guide means for controlling the running of the magnetic tape and a flange edge for regulating the magnetic tape during recording, reproducing, fast forwarding, rewinding, loading, and unloading. This causes buckling and breaking in some cases. In addition, curls and wrinkles are liable to be generated.
This allows for the production of a reinforced film, which is extended in a longer direction (i.e., tape-running direction) or width direction (i.e., tape-width direction, transverse direction) to produce a plastic film such as a polyester film to use as a substrate film for a magnetic tape. Further, it has been attempted to improve not only the substrate film but also the mechanical strength of the magnetic tape by forming a magnetic layer having a high elastic modulus on the substrate film, that is, a non-magnetic support. However, this is not sufficient, and the thickness of the magnetic tape, which becomes thin, can not effectively prevent buckling and breaking of the tape edge portion.
JP-A-50-45877 (the term "JP-A" as used herein means an unexamined published Japanese patent application) and U.S. Pat. No. 4,804,736 (corresponding to JP-A-62-135339) disclose using products having a high strength as a support. JP-A-50-45877 discloses a biaxially oriented film consisting of polyethylene-2,6-naphthalate having a Young's modulus of 51,000 kg/cm.sup.2 or more in a longer direction and 68,000 kg/cm.sup.2 or more in a width direction. Further, U.S. Pat. No. 4,804,736 discloses a polyethylene-2,6-naphthalate film for a high density magnetic recording having excellent heat resistance with a Young's modulus of 800 kg/mm.sup.2 or more in a longer direction and 600 kg/mm.sup.2 or more in a width direction. However, the importance of the Young's modulus in the magnetic layer has not been recognized.
U.S. Pat. No. 4,187,341 (corresponding to JP-A-53-66202) discloses a magnetic recording tape which has a total thickness of 4.0 to 14.5 .mu.m, a ratio of a non-magnetic support thickness to a magnetic layer thickness of 2:3 to 3:2, a ratio of Young's modulus of the non-magnetic support to that of the magnetic layer of 1:2 to 2:1 and a tensile strength at a 0.5% elongation of d.sup.0.5 /16 kg or more based on the total thickness d (.mu.m) of the magnetic recording medium.
It has been found by this proposal that a mere increase in the Young's modulus of the raw material is insufficient, and the Young's modulus of the magnetic layer is increased as much as possible, while the Young's moduli of the non-magnetic support and the magnetic layer are kept at almost the same value. At the same time, the non-magnetic thickness and the magnetic layer thickness have almost the same value, whereby a magnetic recording tape having practical durability can be obtained. In this proposal, however, the importance of directionality in a longer direction and a width direction is not recognized, and there are insufficient improvements in running durability, such as dropping of scratched powders at the tape edge portion, and insufficient reduction in audio and video output.
In order to solve the problems mentioned above, for example, U.S. Pat. No. 4,561,034 (corresponding to JP-A-58-91528) discloses a magnetic recording medium having a total thickness of 17 .mu.m or less which comprises a magnetic layer on one side of a flexible non-magnetic support of high molecular weight material, and a back layer on the opposite side thereof; wherein an elastic modulus (Young's modulus) of the magnetic layer is higher than that of the non-magnetic support and is 1,200 kg/mm.sup.2 or more in the sum of the elastic moduli in the longer direction and wide direction at a 1% elongation; and wherein an elastic modulus of the back layer is higher than that of the non-magnetic support and is 1,200 kg/mm.sup.2 or more in the sum of the elastic moduli in the longer direction and wide direction at a 1% elongation.
However, the sum of the elastic modulus in the longer direction of the non-magnetic support and the elastic modulus in the width direction thereof was 1,100 kg/mm.sup.2, which is well-known, and the problems of running durability, such as dropping of scratched powder at the tape edge portion, an audio and video output reduction could not be sufficiently solved.
Meanwhile, multilayering of a magnetic layer in a magnetic recording medium has been proposed to improve electromagnetic property. For example, U.S. Pat. No. 4,857,388 (corresponding to JP-A-63-103429) discloses a magnetic recording medium comprising a non-magnetic support having two magnetic layers each containing a ferromagnetic powder having a coercive force of 500 Oe (oersted) or more, which is dispersed in a binder, wherein the Young's modulus of a lower magnetic layer (the first magnetic layer) provided on the non-magnetic support is from 500 to 1,000 kg/mm.sup.2 ; the Young's modulus of an upper magnetic layer (the second magnetic layer) provided on the lower magnetic layer is 1,300 kg/mm.sup.2 or more; and the Young's modulus of the whole magnetic layer is 900 kg/mm.sup.2 or more. This results in a very tough magnetic layer, since the lower magnetic layer is flexible, and the reaction (crosslinking) of a urethane resin (or other resin) with polyisocyanate occurs easily because of the high reactivity of the resin with polyisocyanate. This enables one to obtain a magnetic layer having less reduction of output and excellent durability even after storing over a long period of time.
However, this disclosure also does not indicate the importance of a balance in the Young's moduli of the non-magnetic support and magnetic layer in the longer direction and width direction and can not sufficiently solve the problem of running durability, such as dropping of scratched powders at the tape edge portion and an audio and video output reduction.
The extensive investigations made by the present inventors resulted in the discovery that a relative relationship of the directionality in a magnetic layer strength against that of a non-magnetic support as well as the increase in strength of the non-magnetic support was closely related to these phenomena. That is, it has been found that in a magnetic recording medium in which a conventional support with a high strength is used, an insufficient strength in a magnetic layer versus a high strength in a support and the big difference thereof cause damage to the magnetic layer in running.
This investigation has resulted in a magnetic recording medium having excellent durability which is capable of recording for a longer time.